A well-known noninvasive technique for measuring a patient's blood pressure includes the steps of placing a cuff about the patient's limb, inflating the cuff to a pressure high enough to occlude an underlying blood vessel, and then deflating the cuff to a pressure lower than the patient's diastolic pressure. Measurements taken while the cuff pressure decreases are analyzed to determine the patient's pressure. In the oscillometric blood pressure technique, the measurements taken during cuff deflation include the total cuff pressure and the much smaller pressure fluctuations caused by blood flow surges in the underlying blood vessel. In the ausculcatory technique, the measurements taken include the total cuff pressure and the Korotkoff sounds corresponding to the blood flow surges. A third known technique makes use of an ultrasonic transducer and doppler detection means to detect the blood flow surges.
In one well-known application of the oscillometric technique, the cuff is first inflated to a pressure above the patient's expected systolic pressure, and then slowly deflated over a pressure range inclusive of the patient's systolic and diastolic pressures. During cuff deflation, the total cuff pressure is measured by an appropriate transducer in pneumatic communication with the cuff, and the output signal from this transducer is processed to yield signals corresponding to the total cuff pressure and to the smaller pressure fluctuations caused by blood flow surges. The signal corresponding to the blood flow surges is processed to determine the times during cuff deflation at which the cuff pressure was equal to the patient's systolic and diastolic pressures. The cuff pressures at such times are then displayed or otherwise output as the patient's blood pressure values. Analogous methods are known that make use of ausculcatory or ultrasonic techniques.
Circumstances frequently arise in which it is desirable to periodically monitor a patient's blood pressure over a period of time. Automatic blood pressure devices have therefore been produced that can make blood pressure measurements at a selected frequency (e.g., every five minutes) without operator intervention. The cuff deflation rate of an automatic blood pressure device is an important parameter. If the deflation rate is too high, then the cuff will deflate from systolic to diastolic pressure too rapidly, and unreliable blood pressure values will be produced. If on the other hand the deflation rate is too low, the cuff pressure will remain between systolic and diastolic pressures for an excessive period of time, resulting in patient discomfort.
Prior blood pressure measurement devices have either employed a preselected deflation rate, or have permitted operator selection between two or more rates. Devices using a single, preselected rate cannot be adjusted to suit individual patients. Devices permitting operator selection between two or more deflation rates can be matched to individual patient requirements for a single measurement. However in an automatic device, there is no guarantee that the selected deflation rate will remain suitable as the patient's blood pressure varies over time.